Easily Rocket League. It's always been toxic but it's been much worse since becoming free to play.
Rage quitting
Malicious compliance (AFK, accidental own-goals)
Team mates playing for the opposing team entire games
Passive aggressive quick chats
Overtly aggressive free-form chats (being chastised over every tiny mistake)
The game's competitive ranking systems makes it so each win and loss matters, and people often take their vitriol out on their own team before looking inwards.